Since the first discovery of carbon nanotubes (CNTs), boron nitride nanotubes (BNNTs) have been attracting much attention due to the structural similarity between graphite-like carbon system and hexagonal boron nitride (h-BN) system. BNNTs are isoelectronic analogues of carbon nanotubes which can be made by rolling up single or few layered h-BN sheets. In 1994, BNNTs were predicted by theory as a structural counterpart of CNTs in the h-BN system and successfully synthesized in 1995 by an arc discharge method.
Theoretical or computational studies have demonstrated that BNNTs have excellent properties such as low density with high mechanical strength, electrical insulation with high thermal conductivity, piezoelectricity, unique optical/optoelectronic properties, good radiation shielding ability, and superb resistance to thermal or chemical stresses. Some of those properties are predicted to be comparable to or even superior to those of CNTs. Many novel applications of BNNTs in nanoscience and nanotechnologies are expected.
In spite of the predicted potentials of BNNTs, there have been few experimental studies on the detailed properties of BNNTs, most of them being limited to large diameter BNNTs (>10 nm). To fully take advantage of the distinct phenomena occurring at nanoscales, small diameter BNNTs would be more favorable. The lack of experimental study on BNNTs is primary due to the absence of well-established BNNT synthesis methods, which seems to be more challenging compared to the CNT case. In particular, the absence of an effective method for the large-scale synthesis of small diameter BNNTs is still a prime obstacle in further understanding BNNT characteristics and their real applications. Therefore, in order to expand BNNT applications by fully exploring their unique properties, a reliable synthesis route capable of producing ultrafine BNNTs (<10 nm) at large scales (kg/day) is urgent.
Due to the structural similarity, the initial attempts for BNNT synthesis were made by using various modified version of CNT synthesis methods. BNNTs were produced for the first time by evaporating boron (B) containing electrodes in an arc discharge reactor. Laser vaporization processes have also been developed by irradiating lasers on B containing targets under N2 atmosphere. Although BNNTs have been produced successfully in those approaches, the yield rates are low (mg/h) and the products contained various impurities as well, such as metal nanoparticles and h-BN flakes.
Chemical vapor deposition (CVD) methods have been also investigated. BNNTs were produced on the surface of boride nanoparticles from the decomposition of borazine (B3H3N6). A floating catalyst CVD was reported by using borazine along with a vapor phase metal catalyst of nickelocene. In this process, double-walled BNNTs were exclusively produced. A simple ball milling and annealing method was developed but the most of the products were highly disordered or bamboo-type BNNTs.
Recently, a boron oxide CVD (BOCVD) method has been developed using B powder and metal oxide as a feedstock. In this process, white-colored pure BNNTs were produced for the first time but diameters of the BNNTs produced were on the order of 50 nm. Recent advances in this method allowed the production of small diameter BNNTs by choosing an effective metal oxide. Very recently, a so-called pressurized vapor/condenser (PVC) method has been proposed. Highly crystalline, long, and small diameter BNNTs were produced from B vapor under high pressure nitrogen atmosphere (2-250 atm) but again the yield is no more than a few grams per day, the yield rate demonstrated being about 0.1 g/h.
BNNTs have been also prepared using a DC arc-jet plasma generated from a DC arc discharge plasma torch. A mixture of h-BN powder and Ni/Y catalysts was injected into the plasma plume issuing from a DC plasma torch. The formation of BNNTs was confirmed but BNNTs were found in the limited area of the reactor. A variation on a DC arc-jet plasma apparatus requiring material inlet ports along the length of the plasma plume has also been proposed.
Most processes developed so far, including arc discharge, laser vaporization, ball milling, CVD, and BOCVD method, are basically operated in a batch mode, intrinsically limiting their scalability. Reaction times in those processes are also typically long and the yield rates do not meet the needs of the current market. A fair amount of BNNTs can be produced from the ball-milling process, but the characteristics of BNNTs produced in this method are far from those of small diameter BNNTs which have drawn the most interest from theoretical studies.
The floating catalyst CVD method has potential for large-scale production of BNNTs, however this approach is not favorable in terms of the commercial-scale operation as this process employs toxic chemical agents such as borazine or nickelocene, which also contain carbon impurities. The DC arc-jet method has good scalability however the production of BNNTs in this method is not efficient, being limited only to the region of the periphery of the plasma jet which is not truly continuous.
Most processes described above also use metal catalysts which will require additional purification steps in advance to practical applications, increasing the cost and complexity of the overall process.
The PVC method developed recently seems to have a great potential as long as a steady B evaporator is available such as CO2 lasers, free electron lasers, or electron accelerators. However the initial investment or operation cost for such facilities will be very prohibitive at commercial scales. The high pressure operation around 12 atm (the reported optimum N2 pressure) would be another challenge in scaling up the reactor vessel. Currently the daily yield rate is no more than a few g per day.
There remains a need for an efficient scalable process for producing high purity nanoscale BNNTs.